FIG. 6 is a cross-sectional diagram of a heat exchanger for use in a power plant, illustrating a joint portion between the main body of the heat exchanger and a high-temperature pipe for heating steam. In FIG. 6, reference numeral 60 denotes the shell of the heat exchanger. A large number of heat transfer tubes 61, supported by a pair of tube plates 62, are housed in the shell 60. A low-temperature fluid flows through the heat transfer tubes 61. A high-pressure, high-temperature fluid is introduced from a high-temperature fluid inlet connection 63 into the shell 60. Heat exchange takes place between the high-temperature fluid and the low-temperature fluid flowing through the heat transfer tubes 61.
In such a heat exchanger, a thermal stress acts on a region around the joint between the high-temperature fluid inlet connection 63 and the shell 60. This is because the high-temperature fluid inlet connection 63 thermally expands by exposure to a high temperature while the shell 60 is kept at a low temperature, and therefore the joint between the high-temperature fluid inlet connection 63 and the shell 60 is subject to a high compressive stress due to simultaneous occurrence of expansion and contraction at the joint. It is, therefore, conventional practice to employ a thermal sleeve structure in the high-temperature fluid inlet connection 63 to reduce thermal stress.
The above prior art techniques employ a thermal sleeve structure to reduce thermal stress and, in cases where the stress reducing effect is insufficient, provide an insulating means in the thermal sleeve structure to enhance the effect of reducing thermal stress.